U.S. Pat. No. 5,862,054 describes a process parameter monitoring system for real time process control having the capability to monitor multiple process machines, i.e., equipment, at the same time with a mix of different type of machines. The system is applicable to semiconductor wafer process machinery in a foundry fab, and is applicable to other processes and process machinery where it is necessary to for automatic collection of process parameter data for statistical process control, SPC.
Data is stored by a production control computer. The data is used to determine process control limits as well as trends in the characteristics of the process. The process control computer is connected to a company wide network through a server. Through the company wide network the process parameter monitoring system communicates with a production manufacturing system or manufacturing execution system, MES.
Each process machinery is providing key process parameter data to be applied to a real time SPC analysis. The data for each machine is accumulated separately and applied to its own SPC analysis. The data once analyzed is presented on the screen of the production control computer through a graphical users interface, GUI. There is also an alarm indicator when the process is out of control, and an accounting for the number of alarms that have occurred over the time interval of data being shown through the GUI.
The process control monitoring system can display historical data on the production control computer. This display of historical data can be important in the analysis of problems leading to poor yield. When debugging a particular process on a particular machine, the monitoring of that machine can be stopped or started without affecting the monitoring of the other process on other process machinery.
An SPC system uses control charts. For example, the use of control charts for an SPC system is disclosed in U.S. Pat. No. 5,586,041. In a factory using an SPC method for controlling complex process steps, numerous control charts are in a corresponding SPC system. For example, there are more than twenty thousand control charts in a foundry fab. Synchronizing the control limits for various charts is highly desired for obtaining optimum performance of different equipment, i.e., machines, in the foundry fab. Synchronizing control limits for various charts is an arduous task, because these charts are numerous, and are different chart types assigned to controlling different processes.
The control limits need to be verified by statistical support before assigning the control limits to be synchronized. Prior to the invention, a control limit was based on data from a single equipment. The single equipment was not always equal in performance with other equipment that have the same process capability. Thus the control limit of the single equipment could be arbitrarily assigned for the other equipment without statistical support for doing so. Further, control limits were calculated as (+/−3) Sigma of past data. The control limit assigned in this manner may be set too tight for other equipment in the foundry lab. False alarms will result, causing wasted debugging efforts. The control limit assigned in this manner may be set too loose for other equipment in the foundry fab. When the control limit is set too loose, fault detection will occur too late to avoid rework and product scrap. Thus, arbitrarily assigned control limits would not be based on statistics gathered from equipment and processes.
Prior to the invention, a method and a system was needed for synchronizing control limits of various charts for optimum equipment performance. The control limits needed to be updated with calculations based on statistics formed by new data.